For me, the main difference is that Belknap was a Presidential Appointee, someone who has no fixed term of office. To allow such a person to avoid a penalty, by choosing the time of their leave of office, especially ahead of an impeachment vote, would indeed encourage corruption.

Trump, on the present hand, being elected for a fixed term, was removed from office Constitutionally, at the end of his term, negating the need for impeachment.

Very nice write up, and incisive